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Climate and Viral Illness: El Niño Event Linked to Dengue Epidemics

Caption: Incidence of dengue fever across Southeast Asia, 1993-2010. Note increasing incidence (red) starting about June 1997, which corresponds to a period of higher temperatures driven by a strong El Niño. At the end of the El Niño event, in January 1999, dengue incidence is much lower (green). Credit: Wilbert Van Panhuis, University of Pittsburgh

Just as the severity of the winter flu fluctuates from year to year in the United States, dengue fever can rage through tropical and subtropical regions of the world during their annual rainy seasons, causing potentially life-threatening high fever, severe joint pain, and bleeding. Other years—for still unknown reasons—dengue fizzles out. While many nations monitor the incidence of dengue within their borders, their data aren’t always combined to track outbreaks across wider regions over longer times.

Now, NIH-funded researchers and colleagues, reporting in Proceedings of the National Academy of Sciences [1], have linked an intense dengue epidemic that struck eight Southeast Asian countries starting in mid-1997 to high temperatures driven by the strongest El Niño event in recent times. El Niño is a complex, irregularly occurring series of climate changes in the Pacific Ocean with a global impact on weather patterns. This new insight into climatic factors associated with dengue transmission could enable better prevention measures, which may soon be needed because climatologists are predicting another strong El Niño event next year due to unusually high ocean temperatures in the equatorial Pacific.Dengue is a viral infection transmitted by Aedes (meaning “odious” in Greek) mosquitoes. The infection, usually lasting about a week, is rare in the continental United States and typically associated with travel to tropical and subtropical regions. There, “breakbone fever,” as dengue is sometimes called, causes an annual estimated 50 to 100 million infections and 22,000 deaths, mostly among children. The incidence of dengue has jumped 30-fold over the last 50 years, making it one of the most important mosquito-borne viral diseases in the world [2].

In the study, an international team of researchers, led by Wilbert van Panhuis at the University of Pittsburgh Graduate School of Public Health, were interested in why dengue infection rates soar every two to five years. To find clues, the team gathered monthly reports on the incidence of dengue fever over an 18-year period, totaling 3.5 million reported infections in 273 provinces in eight Southeast Asian countries—Thailand, Cambodia, Laos, Malaysia, Singapore, the Philippines, Vietnam, and Taiwan.

Those data showed a strong simultaneous development, or synchrony, in dengue transmission across the entire region, most notably in 1997 and 1998 (the last major El Niño event). They further found that the rise and fall of dengue infection rates from 1997 to 2001 tracked well with the Oceanic Niño Index, a measure of the departure from normal sea surface temperature in the east-central Pacific Ocean. The findings suggest that large dengue epidemics may occur once temperatures rise above a certain level, enhancing the ability of Aedes mosquitoes to reproduce and transmit dengue-causing viruses.

In addition to the higher temperatures associated with El Niño, researchers said other factors were likely at play to generate a “perfect storm” for dengue infection in 1998. A new strain of dengue virus emerged in Asia that same year, leaving more people susceptible to the illness. Many of those who became ill eventually developed immunity to the new strain, contributing to a period of extremely low rates of infection in the following years.

Interestingly, the analysis shows that large cities can drive their own “signature patterns” of infection due to dense populations, which can sustain ongoing transmission of the virus. The researchers also observed traveling waves of dengue infection during those larger outbreaks in parts of Thailand, Laos, and the Philippines, most likely reflecting the spread of the virus as people moved from one area to another.

The findings show the value of extracting and assembling information from public health data. As researchers gear up to monitor dengue infections during the strong El Niño event forecast for 2016, they are hoping for regional collaborations and continuous data sharing to improve the prediction and assessment of infectious disease epidemics.

About 2.5 billion people now live in areas of the globe at risk for dengue fever [3]. Unfortunately, no vaccine is available to prevent the infection, although some experimental vaccines under development are showing promise [4]. Currently, mosquito control measures—indoor spraying and draining stagnant water in which the insects breed—remain the best ways to limit infections.

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About the NIH Director

Francis S. Collins, M.D., Ph.D., was appointed the 16th Director of NIH by President Barack Obama and confirmed by the Senate. He was sworn in on August 17, 2009. On June 6, 2017, President Donald Trump announced his selection of Dr. Collins to continue to serve as the NIH Director.